Mid-Term Marking

Question 1

1 mark maximum for the numerical results (correct guaranteed value and excess) (total of 5 marks). 0.5 marks were subtracted for each missing/wrong value and/or for not explicitly stating that a requirement was met.

Some common mistakes were: confusing the CPU and chip WR* signals and adding two timing specifications that don't end/start at the same point.

Question 2

A maximum of 13 marks were deducted for mistakes in the truth table (TT) portion with marks subtracted as follows:

• 1.5 marks subtracted for each missing or incorrect condition (row) in the truth table. This includes mistakes such as changing state or turning on an enable when CS* is not enabled, turning on an enable when WR* is asserted, not changing or resetting the state on WR*, etc. For rows with don't-care conditions (X's) marks were subtracted for each applicable row (e.g. an incorrect row with 2 X's lost 4*1.5=6 marks). In some cases the number of applicable rows depended on the number of states used. See note 1 below for more details on missing rows.
• 2 marks subtracted for each missing column in the truth table

• 1 mark subtracted for each missing term in an expression with a maximum of 2 marks deducted for errors in each enable (E1 and E2) expression and a maximum of 3 marks deducted for errors in the next-state (*.D) expressions
• 0.5 or 1 mark subtracted for obvious syntax errors depending on how severe I considered the error (e.g. -1 for using over-bars on variable names or -0.5 for leaving out ".D" qualifiers when assigning to latch inputs or not having a clock input.)
• 0.5 mark subtracted for missing/extra negation (!) in an expression.
• marks were not subtracted if the CUPL expressions matched the TT (even if the TT was wrong) unless the TT was missing or trivially short.

Notes:

(1) A truth table must include an entry for each possible input condition for every state used -- there is no universal ``default'' for output or next-state values. You can check your answer by counting the number of lines in the TT and make sure it matches the number required (states*2^(number of inputs)).

(2) Avoid using a don't care (X) unless you are sure it applies. Many people used X's as a shortcut and instead wound up losing a lot of marks. Similarly, avoid using X as an output value and don't simplify expressions unless you are sure of the answer.

(3) A design where the outputs (E1*, E2*) are also the state variables is a Moore state machine and cannot meet the requirements for the device described in this question. The design method taught in this course applies to both Moore and Mealy state machines. People who used Moore state machines generally received very low marks for the first part of this question.

Note: To avoid the above problems in the final exam you will have to use truth values (1=asserted) in your truth tables, you will not be allowed to use don't-cares and you will have to explicitly state the output for each input/state combination.

Question 3

(a) 2 marks for each question. But note the following:

• For part 2 I gave 0 (zero) marks if you gave your answer in terms of 0 and 1 instead of H and L. Not only did the question explicitly ask for the signal level as H/L but by this point in the course you should know that 0 doesn't always mean L.
• For part 3 I subtracted 1 mark if you simply called it a spurious read cycle or explained what was happening but didn't explicitly say that it was an interrupt acknowledge cycle.

(b) 2 marks were given for each of the following actions that your code performed correctly:

• had an ORG statement (I usually forgot to check the address)
• saved and restored the registers that were used (2 marks for saving, 2 marks for restoring)
• incremented the pointer at $1000
• copied a byte from the I/O port to memory
• returned with an RTE instruction

No marks were assigned/deducted for comments.

The most common error in this question was probably using too many or two few levels of indirection on the ``pointer'' variable.

If you think your exam was not marked correctly

Please do not tell me I marked someone else differently and that therefore you should get marked the same way. The above marking criteria applies to everyone. If I didn't mark someone else's exam this way then I made a mistake marking their paper, not yours.